When a water that requires treatment is subjected to a filtration treatment over an extended period of time using a sand filtration device or a membrane filtration device including a separation membrane, impurities that cannot be removed by physical cleaning with air or the like tend to adhere to and accumulate on the membrane surface, within the membrane, or on the sand surface or the like, and therefore cleaning using chemicals need to be used to dissolve and remove these impurities.
Examples of the method used include a method in which the device is operated while a chemical agent such as hypochlorous acid or sodium hydroxide is added to the raw water or the backwash water, and a method provided with a chemical agent cleaning step or the like in which the membrane or the like is immersed periodically in a chemical solution containing a chemical agent (for example, see Patent Documents 1 to 3). The chemical agent cleaning step generally includes a step of dissolving or removing the impurities with a chemical agent, and a flushing step of washing away any chemical agent retained inside the membrane module.
Further, particularly in those cases when a water to be treated that contains organic substance is subjected to a filtration treatment, an oxidizing agent such as hypochlorous acid is typically added to the raw water or the backwash water for the purpose of inhibiting slime generation.
In the types of methods described above, when an oxidizing agent such as hypochlorous acid is added to the raw water or the backwash water, or an oxidizing agent cleaning step is performed, residual oxidizing agent is retained in the filtration-treated water, and if a reverse osmosis membrane treatment using a reverse osmosis membrane is performed in a later stage, the oxidizing agent enters the reverse osmosis membrane treatment device. Polyamide reverse osmosis membranes, which are generally in widespread use, are sensitive to oxidizing agents such as hypochlorous acid, and the reverse osmosis membrane tends to degrade upon exposure to oxidizing agents.
Accordingly, it has been necessary to add a reducing agent or use activated carbon or the like to remove any residual oxidizing agents prior to the reverse osmosis membrane treatment. Moreover, in order to inhibit slime generation on the reverse osmosis membrane, an additional slime control agent (antibacterial agent) also needs to be added.
For example, FIG. 2 is a schematic view illustrating a conventional filtration treatment system. In the conventional filtration treatment system illustrated in FIG. 2, a water to be treated that contains organic substance is stored in a raw water tank 50, a prefiltration treatment is then performed in a membrane filtration device 52 using a separation membrane, the prefiltration-treated water is stored in a prefiltration-treated water tank 54, an activated carbon treatment is then performed in an activated carbon column 58, and following storage of the activated carbon-treated water in an activated carbon-treated water tank 60, a reverse osmosis membrane treatment is performed in a reverse osmosis membrane treatment device 56 to obtain a concentrate and a permeate. When cleaning of the membrane filtration device 52 becomes necessary, hypochlorous acid is supplied as a cleaning chemical agent to at least a portion of the prefiltration-treated water from the prefiltration-treated water tank 54, and backwashing is performed by supplying the resulting solution to the membrane filtration device 52 as a backwash water. The hypochlorous acid remaining in the prefiltration-treated water is removed in the activated carbon column 58, and an additional slime control agent (antibacterial agent) is added upstream from the reverse osmosis membrane treatment device 56 to inhibit slime generation on the reverse osmosis membrane.
Further, in those cases where an oxidizing agent is injected into the backwash water or an oxidizing agent cleaning step is performed periodically, in order to ensure that the oxidizing agent is washed satisfactorily out of the membrane module, a large amount of wash water need to be used in the flushing step, and other problems also arise, including generation of a large amount of washing wastewater containing the oxidizing agent, and the need to use an another oxidizing agent to neutralize the washing wastewater, resulting in increased costs for wastewater treatment.